It is known in the plastic molding art to use pressurized fluid in conjunction with the plastic molding of articles. The pressurized fluid is typically nitrogen gas which is introduced into the mold cavity with the molten plastic. An example of such a use is shown in U.S. Pat. No. 4,101,617.
The pressurized fluid serves a number of purposes. First, it allows the article so formed to have hollow interior portions which correspond to weight and material savings. Second, the pressurized fluid within the mold cavity applies outward pressure to force the plastic against the mold surfaces while the articles set. This is desirable toward enhancing surface quality by minimizing sink marks in areas of the article having relatively thicker plastic sections, such as structural ribs. Other advantages include: reduced stress and warpage for painting; better control on knit line location; larger parts on presses with reduced tonnage; reduced time cycles for thick sections; eliminate hot runners; good surface finish; and reinforcement with tubular section.
However, the added equipment and process control parameters required to implement fluid injection contribute significantly to the cost and complexity of this type of injection molding.
Another concern is that when the gas and plastic are injected into the mold from the same nozzle, the gas gets trapped in the nozzle area thereby foaming any molten resin in that area during venting. This foamed plastic causes blemishes (i.e. splay) in the next part when injected into the mold.
Of particular concern are the fluid charging components and circuits needed to charge, inject and vent the pressurized fluid at the controlled times and pressures necessary for quality part production on a repeatable, volume basis. Prior art devices addressing this concern have answered with complex valves and seals which are expensive, difficult to operate, and require maintenance or replacement at frequent intervals. The problem is especially acute with seals whose performance diminishes over their useful life.
Examples of the injection of pressurized fluid separate from the plastic injection nozzle are illustrated in U.S. Pat. Nos. 4,935,181; 4,555,225; 4,740,150; and 4,942,006. While it is often desirable to introduce the pressurized fluid as close to the article-defining cavity as possible to shorten the path of the pressurized fluid through the plastic resin, one problem of many of such devices is that the aperture through which the pressurized fluid flows can become clogged with molten plastic. Another problem is uncontrolled pressurized fluid leakage about the pin through which the pressurized fluid is introduced into the article in the mold.
U.S. Pat. No. 4,943,407 discloses a method, system and device which are provided for the injection molding of plastic articles using a pressurized fluid, normally nitrogen gas, wherein the gas is prevented by the device from contaminating the molten resin in the nozzle utilized therein. In one embodiment of the '407 patent, a rotary valve is utilized in the device which is located in the sprue area of the mold. In one position of the valve, molten plastic flows therethrough. In another position of the valve, gas is communicated into the sprue with the valve blocking the flow of plastic.
Published French patent document No. P.V. 8912991 entitled "Injection of Gas Under Pressure Into A Fabrication Mold Coming As A Single Piece Of Plastic Material, The Mold And Process Using Such An Injector, And The Plastic Part Obtained" discloses a fixed gas injector which projects into the mold cavity. Gas is injected into the cavity through a very small diametral play between a sheath and a needle of the injector.